Tracing an image with electrons

March 5, 1998: Light microscopes are limited in how much small detail they can see because as the magnification goes up, the sharpness and brightness of the picture go down.

A scanning electron microscope uses a different principle. Like a rapid-fire BB gun, it scans electrons across the target which reflect onto a fluorescent screen where the image is captured by a camera and enlarged. Electrons are much smaller than atoms, so a scanning electron microscope paints a razor-sharp image of the target. This is useful for mapping details of objects that light microscopes can see. The downside is that the picture has no true colors, just black and white.

The Environmental Scanning Electron Microscope (ESEM; shown above) in MSFC's Materials and Processes Laboratory is the latest advance in electron microscopy by sidestepping the need to contain the sample in a "hard" vacuum.

Most scanning electron microscopes require a hard vacuum in the sample chamber for two reasons.

First, the electron beam, used to image the sample, will be scattered by any gas molecules in the chamber. This is analogous to shining a bright light in the fog. The denser the fog, the more the light is scattered and the less you can see in front of you. Second, the electron signals produced by the sample due to the electron beam must be seen by an electron detector. This detector is analogous to your eyes. As you drive in fog, your headlights must not only penetrate the fog but the reflected light from another car coming at you must be bright enough for your eyes to see.

To eliminate the problem of electron scattering in low vacuum, the ESEM uses two new technologies.

The first technology is a differential vacuum pumping system. This means the electron gun and column where the electron beam is created and focused is at high or "hard" vacuum. The ESEM has two small pumping chambers between the low vacuum sample chamber and the high vacuum electron optic column. The electron beam is thus only scattered by gas molecules over a very short distance.

The other technology is a patented environmental electron detector. The sample chamber is backfilled with a small amount of water vapor and operates at a pressure of 1 to 10 torr, as opposed to 0.0001 torr, the maximum that normal scanning electron microscopes can tolerate. The water vapor is ionized by the electron signal generated on the sample. The ionized gas is then detected by the environmental detector.

ESEM can show extraordinary details in the ordinary (from left): A refugee from Independence Day is just a

This system has two major advantages over normal scanning electron microscopes. The water vapor acts as the electron ground path away from the sample. Thus you can look at non-conductive samples such as rocks or biological tissue samples without the need for adding a conductive coating (samples usually are coated by a whiff of gold vapor to provide this coating). And, the electron signal produces more than one ion per electron, so the signal is amplified without extra electronics which can add noise to the image.

The ESEM can magnify the view as much as 100,000 times. The electrons colliding with the target generate X-rays which an energy dispersive spectrometer spreads much like a grating spreads light into a rainbow. This identifies chemicals as light as boron, just one notch below carbon on the periodic table.

The ESEM was originally designed for looking at biological organisms, even in water. The investigators plan to examine living microorganisms in their natural hydrated state for comparison to the ongoing findings.

Air pressure is measured in torr, named after Evangelista Torricelli who invented the barometer in 1643. Torricelli demonstrated that air pressure will support a column of mercury in an upside down vacuum tube. At sea level, the column is pressure is about 760 mm (29.92 in.) tall. We now take 760 mm, or 760 torr (also called bar), as standard pressure. Most SEMs work at 0.0001 torr - 1/7,600,000 of sea level atmospheric pressure. A perfect vacuum is impossible to achieve, even in deepest space, so we measure air pressure in small fractions of a torr.

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